A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, such as hydrogen or an alcohol, such as methanol or ethanol, is supplied to the anode and an oxidant, such as oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but ionically conducting. In the PEM fuel cell the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.
The principle component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Conventionally, the MEA can be constructed by a number of methods outlined hereinafter:
(i) The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of an ion-conducting membrane and laminated together to form the five-layer MEA;
(ii) The electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst coated ion-conducting membrane. Subsequently, gas diffusion layers are applied to both faces of the catalyst coated ion-conducting membrane.
(iii) An MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs; removing products; providing electrical connections; and providing physical support.
Conventional ion-conducting membranes used in the PEM fuel cell are generally formed from sulphonated fully-fluorinated polymeric materials (often generically referred to as perfluorinated sulphonic acid (PFSA) ionomers). Membranes formed from these ionomers are sold under the trade names Nafion® (e.g. NR211 or NR212 from E.I. DuPont de Nemours and Co.), Aciplex™ (Asahi Kasei) and Flemion® (Asahi Glass KK). Other fluorinated-type membranes include those sold under the trade name Fumapem® F (e.g. F-930 or F-1030 from FuMA-Tech GmbH), Aquivion™ from Solvay Solexis S.p.A and the GEFC-10N series from Golden Energy Fuel Cell Co., Ltd.
As an alternative to perfluorinated, and partly-fluorinated, polymer based ion-conducting membranes it is possible to use ion-conducting membranes based on non-fluorinated sulfonated or phosphonated hydrocarbon polymers, and in particular polyarylene polymers. Such commercially available polymers include Udel® polyarylenesulfone (PSU) and Veradel® polyarylene ether sulphone (PES) from Solvay Advanced Polymers, and Victrex® polyarylene ether ether ketone (PEEK™) from Victrex plc. Hydrocarbon polymer based membranes are also described, such as the Fumapem® P, E and K types from FuMA-Tech GmbH., JHY and JEM membranes from JSR Corporation, SPN polymer from Toyobo Co., Ltd., and developmental membranes from Toray Industries Inc.
In PEM fuel cells designed to operate at higher temperatures (e.g. 150° C. to 190° C.), the membrane may be a polymer such as polybenzimidazole, or polymer matrix, impregnated with phosphoric acid. Examples of MEAs made from such membranes include the Celtec®-P series from BASF Fuel Cell GmbH. Other MEAs include the Advent TPS® series based on aromatic polyether polymers incorporating pyridine type structures, also impregnated with phosphoric acid, from Advent Technologies S.A. Polybenzazole polymers can also be used such as ayrl or alkyl substituted polybenzimidazole (e.g. polybenzimidazole-N-benzylsulfonate), polybenzoxazoles and polybenzothiazoles.
The PFSA or hydrocarbon based ion-conducting membrane may contain a reinforcement, typically wholly embedded within the membrane, to provide improved mechanical properties such as increased tear resistance and reduced dimensional change on hydration and dehydration. The preferred reinforcement may be based on, but not exclusively, a microporous web or fibres of a fluoropolymer such as polytetrafluoroethylene (PTFE), as described in U.S. Pat. No. 6,254,978, EP 0814897 and U.S. Pat. No. 6,110,330, or polyvinylidene fluoride (PVDF), or alternative-materials-such as PEEK or polyethylene.
Conventionally, electrocatalyst layers are formed using well-known techniques, such as those disclosed in EP 0 731 520. The catalyst layer components may be formulated into an ink, comprising an aqueous and/or organic solvent, optional polymeric binders and optional proton-conducting polymer. The ink may be deposited onto an electronically conducting gas diffusion layer or an ion-conducting membrane using techniques such as spraying, printing and doctor blade methods.
The anode and cathode gas diffusion layers are suitably based on conventional gas diffusion substrates. Typical substrates include non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series from Ballard Power Systems Inc, or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into a MEA either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of fuel cell and the operating conditions that will be used. The substrate can be made more wettable by incorporation of materials such as amorphous carbon blacks via impregnation from liquid suspensions, or can be made more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylenepropylene (FEP), followed by drying and heating above the melting point of the polymer. For applications such as the PEMFC, a microporous layer may also be applied to the gas diffusion substrate on the face that will contact the electrocatalyst layer. The microporous layer typically comprises a mixture of a carbon black and a polymer such as polytetrafluoroethylene (PTFE).
Typical electrocatalysts are selected from                (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium),        (ii) gold or silver,        (iii) a base metal,        
or an alloy or mixture comprising one or more of these metals or their oxides. The metal, alloy or mixture of metal may be unsupported or supported on a suitable support, for example particulate carbon. The electrocatalyst most appropriate for any given electrochemical device would be well known to those skilled in the art.
It has been found that using such components and constructing the MEA in such a manner can lead to a number of problems including cracking of the catalyst layers, which can lead to increased gas crossover, peroxide formation and thus increased membrane degradation; delamination at the catalyst layer to membrane interface and other mechanical failures due to expansion and contraction during wet and dry cycles experienced by the MEA.